Realizing Scalable Conditional Operations through Auxiliary Energy Levels

TL;DR

This paper introduces a novel auxiliary energy level-based scheme for scalable conditional quantum operations, reducing circuit complexity and improving fidelity in NISQ devices, enabling more practical large-scale quantum algorithms.

Contribution

It presents a transition composite gate scheme leveraging auxiliary energy levels for digital implementation of conditional operations, demonstrated experimentally with significant circuit depth reduction.

Findings

Reduced circuit depth for GHZ and W state preparation by 40-44%.

Fidelity improvements of 1.5% and 4.2% in entangled state preparation.

Successful implementation of a quantum comparator with 72% reduced circuit depth.

Abstract

In the noisy intermediate-scale quantum (NISQ) era, flexible quantum operations are essential for advancing large-scale quantum computing, as they enable shorter circuits that mitigate decoherence and reduce gate errors. However, the complex control of quantum interactions poses significant experimental challenges that limit scalability. Here, we propose a transition composite gate scheme based on transition pathway engineering, which digitally implements conditional operations with reduced complexity by leveraging auxiliary energy levels. Experimentally, we demonstrate the controlled-unitary (CU) family and its applications. In entangled state preparation, our CU gate reduces the circuit depth for three-qubit Greenberger-Horne-Zeilinger (GHZ) and W states by approximately 40-44% compared to circuits using only CZ gates, leading to fidelity improvements of 1.5% and 4.2%, respectively. Furthermore, with a 72% reduction in circuit depth, we successfully implement a quantum comparator-a fundamental building block for quantum algorithms requiring conditional logic, which has remained experimentally challenging due to its inherent circuit complexity. These results demonstrate the scalability and practicality of our scheme, laying a solid foundation for the implementation of large-scale quantum algorithms in future quantum processors.